Cereals are plants of the grass family. The many separate dry fruits they produce are grains. Certain cereal grains are edible. Used as a food since the earliest times, cereal grains have been hugely important in human history. Their small bulk and excellent keeping qualities made them a crucial survival food. As the first plants to be cultivated, their domestication marked the transition of humans from hunter-gatherers to agriculturalists and enabled the foundation of civilizations. Cereal grains continue to be essential human fare and today constitute the single most important class of food in the world.

As a plant's embryonic offspring, packaged with food for its development, cereal grains are concentrated sources of nourishment. They contain protein and either carbohydrates or fats. However, all grains are deficient in one or more of the essential amino acids, making them an incomplete protein source for animals.

All grains have the same basic structure. Beneath the protective fibrous outer layers, collectively called 'bran', is the endosperm, which comprises most of the grain's volume and stores most of its carbohydrate and protein. At the base of the endosperm is the oil-rich embryo or 'germ'. Both the bran and the germ contain B-group vitamins and minerals.

Cereals are principally consumed cooked; as grain, they are often eaten in the form of porridge, or as various bread doughs, ground into flour.

Although cereals have many common characteristics, their individual differences have resulted in diverse culinary traditions. Staple cereals differ with geographic zones. Their adatability to local conditions, yield, and comparative culinary qualities, determined their distribution, as cereals native to one region were introduced elsewhere.

Wheat became the foremost cereal due to the unique quality of its protein. The gluten that forms when wheat flour is mixed with water both resists and stretches under pressure. Because it can expand to accommodate gases produced by yeast, yet contain them, wheat breads can be leavened (caused to rise).

Pseudo-cereals, while not grasses, have a similar nutrient composition to cereals and are treated as such.